Formation of Chitosan-Based Hydrogels Network - Chitosan Based Hydrogels: Functions and Applications - page 201

mobility of water in the semi-IPN hydrogels vary with the change in water content, which
is also responsible for the alteration of free volume and the diffusion coefficient [153].
A comparative study of the swelling behavior of cr-CS and its semi-IPN with poly(oxy-
propylene glycol) (cr-CS/PE semi-IPN) was also carried out by using DSC, NMR, and
positron annihilation lifetime spectroscopy. The results reveal that cr-CS/PE semi-IPN-
containing hydrophobic moieties have more free volume for water at definite water con-
tent and restricts its mobility less, and the water clusters coexist with water molecules
hydrogen-bonded to the network at the beginning of swelling [154].
The swelling and mechanical performance of a chitosan/PEO semi-IPN and a reference
chitosan gel were evaluated and compared by Agnely and coworkers [155]. From the
swelling study it was concluded that the semi-IPN has a promising potential because of
its higher pH-dependent swelling properties, which could allow a pH-controlled release
of drug in oral administration. The DSC measurements indicate that the semi-IPN con-
tained 6% more bound water than the reference gel, probably due to the presence of the
hydrophilic PEO chains. As regards the mechanical properties, which are a key parame-
ter for the potential use of the system as an implant, the presence of the PEO physical
network enlarges the elastic character for the semi-IPN compared to the chitosan refer-
ence gel.
Shyu and coworkers [156] found a route via polymer blend and cross-linking/grafting
modification to obtain novel biomaterials by interpenetrating a natural polymer modified
polysaccharide, 6- O -carboxymethylchitosan (6-OCC) with various waterborne polyure-
thane (WPU) chains. Semi-IPN membranes are prepared by cross-linking/grafting the
6-OCC/WPU composites with GA or with ethylene glycol diglycidyl ether. The results
reveal that the miscibility of a 6-OCC/WPU composite membrane is increased after being
converted into a semi-IPN membrane, and the antibacterial capability and thrombo resis-
tance of the WPUs are significantly improved via the formation of the semi-IPN mem-
branes. The results of this work suggest that the 6-OCC/WPU semi-IPN membranes may
be used as a biomaterial for blood-contracting devices.
For tissue-engineering applications, it is expected that the homogeneous blending of a
hydrophilic polymer with the hydrophobic polycaprolactone (PCL) chains improves water
diffusion to the proximities of PCL chains, thus accelerating their hydrolytic degradation.
PCL and chitosan (CHT) are immiscible polymers. Apparently homogeneous cosolutions
are obtained mixing PCL solution in pure acetic acid with chitosan solutions in 1 M acetic
acid for CHT/PCL weight ratios up to 30:70. Then, a PCL/CHT semi-IPN is prepared by
simultaneous precipitation and CHT cross-linking with tripolyphosphate. High-porosity
PCL/CHT scaffolds with open pore structure and good interconnectivity are also
obtained. Mechanical properties evaluated by dynamic-mechanical analysis decrease as
porosity increases. The physical interactions between functional groups of CHT and car-
bonyl groups of PCL are assessed by FTIR, the shifting of the main relaxation of PCL
toward high temperatures as the fraction of CHT increases as well as the evolution of the
thermal properties of the system [157].
To overcome the fast dissolution of Poloxamer (P) gels, improve swelling properties and
sustain the release of 5-FU for longer, P gels with an interpenetrating chitosan network
(CS network) that is cross-linked by GA, P-CS/GA gels were developed by Chung et al.
[158]. The results indicated that the swelling ratios of all P-CS/GA gels are markedly supe-
rior to those of nonswelling P and P-CS gels. In vitro releases of 5-FU from P-CS/GA gels
have significantly lower initial burst release and last much longer than those from gels
without a CS network. The release of drugs from gels with an interpenetrating CS network
can be modeled by Fickian diffusion; the characteristic constant k of drug-gel systems